The present invention relates to systems employing multi-tone modulation such as, e.g., OFDM digital communication systems.
Orthogonal Frequency Division Multiplexing (OFDM) is an increasingly important technique in digital communications. OFDM provides excellent performance in communication environments where a signal takes multiple paths from a transmitter to a receiver. OFDM finds application in both wireless environments and wireline environments such as digital subscriber line (DSL) applications. Some OFDM implementations in the DSL environment are referred to as discrete multi-tone (DMT).
In OFDM, the available bandwidth is effectively divided into a plurality of subchannels that are orthogonal in the frequency domain. During a given symbol period, the transmitter transmits a symbol in each subchannel. To create the transmitted time domain signal corresponding to all of the subchannels, an IFFT (Inverse Fast Fourier Transform) is applied to a series of frequency domain symbols to be simultaneously transmitted, a xe2x80x9cburst.xe2x80x9d The frequency domain symbols are also referred to as tones.
For this reason, OFDM is sometimes referred to as multitone modulation. The resulting series of time domain symbols is augmented with a cyclic prefix prior to transmission. The cyclic prefix is a duplicate of the last v time domain symbols of the time domain burst. The cyclic prefix addition process may be characterized by the expression:
[z(1) . . . z(N)]Txe2x86x92[z(Nxe2x88x92v+1) . . . z(N)z(1) . . . z(N)]T
On the receive end, the cyclic prefix is removed from the time domain bursts. An FFT is then applied to recover the simultaneously transmitted frequency domain symbols. The cyclic prefix length vis greater than or equal to a duration of the impulse response of the overall channel. Use of the cyclic prefix assures orthogonality of the frequency domain subchannels in the multipath environment. FIG. 1 depicts an N symbol long time domain burst with a v length cyclic prefix that duplicates the last v symbols of the time domain burst.
FIG. 2 depicts a prior art OFDM transmitter system. A frequency domain burst development block 202 receives data as input and outputs frequency domain symbols. This block includes, for example, error correction coding, interleaving, and bit to symbol mapping as are known in the art. Block 202 generates a series of N symbol long bursts of frequency domain symbols. An IFFT block 204 accepts such bursts and transforms them into the time domain using the IFFT procedure. A buffer 206 receives each time domain burst as it is generated by block 204 and stores it for N symbols periods, i.e., a burst. The cyclic prefix is appended to each burst by the operation of a multiplexer 208. After Nxe2x88x92v symbols of the burst have been loaded into buffer 206, multiplexer 208 selects input from buffer 206 representing the last v symbols of the time domain burst to output as the cyclic prefix. Multiplexer 208 then switches to obtain all N symbols of the time domain burst in order and these are output after the cyclic prefix is output.
Problems arise in the practical implementation of the scheme of FIG. 2. Implementing buffer 206 requires a significant amount of high speed storage on the same integrated circuit that implements IFFT block 204. Furthermore, buffer 206 introduces latency lasting one burst period into the overall OFDM communications link. This additional latency may be critical in, for example, a system that is implementing real time traffic such as voice data or video conference data. What is needed are systems and methods for appending cyclic prefixes to OFDM bursts while introducing less latency and requiring less memory than previous techniques.
Systems and methods for appending cyclic prefixes to OFDM bursts while adding minimal latency and requiring minimal additional memory are provided by virtue of the present invention. This facilitates lower cost implementations of OFDM communication systems including systems that carry latency intolerant traffic such as telephony and video conferencing. The phases of frequency domain symbols are rotated prior to application of the IFFT so that cyclic prefix addition may be implemented as cyclic postfix addition. Cyclic postfix addition requires much less memory and imposes much less latency than cyclic prefix addition.
According to one aspect of the present invention, in a multi-tone transmitter, a method for developing a multitone signal includes: developing a frequency domain burst of N frequency domain symbols, phase rotating the frequency domain symbols, transforming the frequency domain symbols as phase rotated into a time domain burst, and appending to an end of the time domain burst a cyclic postfix duplicating a first v time domain symbols of the time domain burst.